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Stratotypes and Type of Localities (Philippine Stratigraphic Guide)
Book source Philippine Stratigraphic Guide Chapter 3 A. DEFINITIONS 1. Stratotype (Type Section) Stratotype (type section) is the original or subsequently designated type section of a named layered stratigraphic unit or of a stratigraphic boundary representing a specific interval or point in a specific sequence of rock strata, and constituting the standard for the definition and characterization of the stratigraphic unit or boundary. 2. Unit-stratotype A unit-stratotype is the type section of strata serving as the standard for the definition and recognition of a stratigraphic unit. The upper and lower limits of a unit-stratotype are its boundary-stratotypes. 3. Boundary-stratotype A boundary-stratotype is a specific point in a specific sequence of rock strata that serves as the standard for definition and recognition of a stratigraphic boundary. 4. Composite-stratotype A composite-stratotype is a combination of several specified type intervals of strata, whose components may include the original stratotype designated by the original author (holostratotype) and other kinds of sections distinguished by the prefixes para-, lecto-, neo-, and hypo- (see below). 5. Type Locality and Type Area The type locality of a stratigraphic unit or of a boundary between layered units is the specific geographic locality in which the unit-stratotype or the boundary-stratotype is situated; or lacking a designated stratotype, the locality where the unit or boundary was originally defined or named. The type area is the geographic area or region that encompasses the stratotype or type locality. Note 1. Type section is never changed. The definition and name of a lithostratigraphic unit are established at a type section (or locality) that, once specified, must not be changed. If the type section is poorly designated or delimited, it may be redefined subsequently. If the original specified stratotype is incomplete, poorly exposed, structurally complicated, or unrepresentative of the unit, a principal reference section or several reference sections may be designated to supplement, but not to supplant, the type section. 6. Holo-, Para-, Neo-, Lecto- and Hypostratotypes a. Holostratotype – The original stratotype designated by the original author. b. Parastratotype - A supplementary stratotype used in the original definition to illustrate the heterogeneity of the stratigraphic unit. c. Lectostratotype - A stratotype for a previously described stratigraphic unit selected later in the absence of an adequately designated original stratotype. d. Neostratotype – A new stratotype selected to replace the older one which has been destroyed, covered, or otherwise made inaccessible. e. Hypostratotype (also called reference section) – A stratotype proposed after the original designation of the holostratotype (and parastratotype) in order to extend knowledge of the unit or boundary to other geographic areas. Holostratotypes and parastratotypes are generally situated within the type area. Neostratotypes and lectostratotypes are preferably chosen within the limits of the original type area. Hypostratotypes may be chosen beyond the limits of the original type area. In principle, stratotypes and type localities should not be altered or amended. There may be more than one typical section or locality but only one stratotype or type locality. B. REQUIREMENTS FOR STRATOTYPES 1. Expression of concept The most important requisite of a stratotype is that it adequately represents the essentials of the concept for which it is the material type. A complete exposure of all strata in the unit from bottom to top and throughout its entire lateral extent would be the ideal stratotype. However, because it is seldom possible to find or establish such a comprehensive stratotype, reliance is usually placed on a single section, as complete and well exposed as possible. Lack of continuous exposures or the presence of structural complications may make it impossible to find such a single continuous section through an entire stratigraphic unit. It may then be necessary to resort to a composite-stratotype or to supplementary reference sections (parastratotypes and hypostratotypes), or to express the unit-stratotype simply as the stratigraphic interval between a designated boundary-stratotype marking the base of the unit and another designated boundary-stratotype marking the top of the unit. 2. Description The description of a stratotype is both geographic and geologic. The geographic description should enable anyone to readily find the stratotype in the field. It should include a detailed map showing location and means of access to the type locality. It would also be desirable to include suitable photographs at an appropriate scale to show the geographic extent of the unit in the type area and the geographic position of its boundaries. The geologic description should include lithology, thickness, structure, distribution, texture and composition, paleontology, geomorphic expression, and other geologic features of the type section. A unit-stratotype should be clearly delimited by boundary stratotypes for its base and for its top. The boundaries and relations with adjacent units particularly should be described in detail, and reasons for choice of boundaries should be given. The description should be accompanied by a geologic map, columnar sections, graphic profiles, structure sections, and photographs. 3. Accessibility If the stratotype is to fulfill the role of a standard, it should be situated in an area that is geographically accessible to all who are interested, and it should offer reasonable assurance of long-range preservation. 4. Subsurface Stratotypes Subsurface stratotypes are acceptable if adequate surface sections are lacking and if adequate subsurface samples and logs are available. Definition of a subsurface stratotype is also justified where the subsurface section differs materially from the laterally equivalent surface section or where there is doubt about the equivalence of a subsurface unit and a surface section.